Frontend: use preferred path spellings for paths

[compnerd]

Convert paths to the preferred spelling before passing it to the SourceManager to ensure that we have the canonical spelling. This is important as a path with a non-canonical path separator would otherwise fail to be looked up in the source manager buffer.

[compnerd]

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CC: @j-hui

[j-hui]

Thanks for getting around to this. I think this is the right thing to do here (and avoids the lifetime issues I encountered in #80564).

However, I'm not convinced that this will close #81012 just yet because it only canonicalizes the path separators from -verify-additional-file. My logging showed that the paths from -verify-additional-file and from the capture error both have inconsistent path separators:

# .---command stderr------------
# | Got additional file: [[42]]::C:\Users\swift-ci\jenkins\workspace\swift-PR-windows\swift\test\Interop\Cxx\foreign-reference/Inputs/inheritance.h
# | Captured error in [[2]]::C:\Users\swift-ci\jenkins\workspace\swift-PR-windows\swift\test\Interop\Cxx\foreign-reference/Inputs\inheritance.h: 125
…

So, this patch should fix the problem for bufID [[42]] here, but not bufID [[2]].

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[compnerd]

I think that fixing [2] is going to require more work and we should do this incrementally. Fixing the first half gets us to a better place as we work towards improving this.

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